one solution, might be getto but that is to have all fields on all templates that will be using custom fields and just have them display:none.
Then just do some checking with jquery on what page template were are on (#page_template) and also check if it changes and just add display:block on field blocks that should be displayed.
Is this really bad? What would the performance be on the dashboard and the db if having let say 90 fields in total.
